1. Field of the Invention
This invention relates to an optical transmission apparatus in a communication system such as an optical frequency-division multiplex system (optical FDM), and a semiconductor laser device suitable for use in the optical transmission apparatus.
2. Description of the Related Art
Tremendous research and development efforts have been directed toward optical communication technology. The reason for this is that optical signal transmission is superior to electric signal transmission in transmission speed and interference between signals. With this backdrop, optical frequency-division multiplex systems (optical FDMs) today are attracting attention. The optical FDM is a communication system which enables a large capacity transmission through multiplexing in optical frequency space, and can be applied to various fields including optical interconnection, optical conversion, and optical arithmetic operation. Particularly, coherent optical communication using optical frequency and phase is capable of high-density frequency multiplexing, resulting in a remarkable increase in the amount of information transmitted. It therefore has a bright future as a next-generation communication system.
Coherent optical communication requires a light source which oscillates in a single longitudinal mode and has a narrow spectral line width. The optical FDM system, in addition to this, requires the light source to be tunable over a wide range of wavelengths. The tunable light source is used in selecting a channel on the reception side. The wider the range over which the light source is tunable, the more channels can be received. To construct an n-to-n optical communication requires a short channel switching time.
For coherent light sources for optical FDM, a compact, reliable, electrically tunable semiconductor laser device is now being developed. To realize a large capacity optical FDM, it is necessary to develop a semiconductor laser device tunable over a wide wavelength range. In addition, to multiplex more and more frequency channels in a specific range of wavelengths, it is necessary to make narrower the frequency range occupied by each channel, i.e., to make the oscillation wavelength spectral width narrower. Especially, in the coherent optical transmission system promising in the field of optical FDM, a light source with a very narrow line width is needed to obtain a reception signal through interference between a signal light and a local oscillation light.
Tunable semiconductor laser devices now being researched are those in which an oscillation wavelength is controlled according to carrier density and temperature, and are classified into four categories:
(1) Multielectrode distributed Bragg-reflector (DBR) semiconductor laser device PA1 (2) Twin guide semiconductor laser device PA1 (3) Temperature-controlled semiconductor laser device PA1 (4) Multielectrode distributed feedback (DFB) semiconductor laser device
With the multielectrode DBR semiconductor laser device in item (1), the wavelength is changed by controlling the injection of current into the Bragg reflection region and the phase matching region and thereby changing the refractive index. Since the carrier density in the passive waveguide (the Bragg reflection region and the phase matching region) can be changed greatly by current injection, the carrier density contributes more to the change of the refractive index in the Bragg reflection region and phase matching region than the temperature. Since the change rate of carrier density is as fast as on the order of nanoseconds, it is possible to realize a short channel switching time using a multielectrode DBR laser.
However, as the amount of carriers injected into the passive waveguide grows larger, the absorption loss increases, with the result that the optical output decreases and at the same time, the spectral line width increases. Because of this, an optical transmission apparatus using a multielectrode DBR laser has a problem: the variable wavelength changing range in which the spectral line width can be kept narrow is smaller.
The twin guide semiconductor laser device in item (2) independently controls a current flowing in an active layer and that flowing in an optical waveguide layer closely stacked with each other. It can be considered to have a region divided in the layer direction instead of the axis direction in a multielectrode DBR semiconductor laser device. Thus, it operates in a similar mode to the multielectrode DBR semiconductor laser device. However, because of the limitation of the oscillation spectral line width, it is difficult to apply the twin guide laser device to coherent optical transmission.
The temperature-controlled semiconductor laser device in item (3) has heating means in the vicinity of the active region to raise the temperature of the active layer. Generally, by raising the temperature of the active layer, the oscillation wavelength can be shifted so as to be longer (red shift), and therefore be changed greatly with the line width kept narrow. However, since a great change of the oscillation wavelength makes the time required for the oscillation wavelength to be stable as long as several milliseconds, it is difficult to apply the device to applications where optical FDM channels are switched at high speeds, such as optical LANs.
The multielectrode distributed feedback semiconductor laser device in item (4) is such that a DFB semiconductor laser device is divided into regions in the direction of the resonator, and the carrier density distribution and the temperature distribution are changed to vary the oscillation wavelength.
FIG. 1 shows a schematic structure of a three-electrode DFB semiconductor laser device as an example of a multielectrode DFB semiconductor laser device.
In the figure, numeral 100 indicates a p-type semiconductor substrate. On the p-type semiconductor substrate 100, an optical waveguide layer composed of an active layer 102 and a diffraction grating 129 is formed. On the optical waveguide layer, an n-type cladding layer 134 and an n-type ohmic contact layer 136 are formed in sequence.
The n-type cladding layer 134 and n-type ohmic contact layer 136 are divided by two grooves 130 into three current injection regions 131, 132, 133. These three current injection regions 131, 132, 133 are provided with electrodes 113a, 113b, 113c, respectively.
In the center of the diffraction grating 129, a phase shift region 137 is formed. In order that end reflection should have no effect in phase on the diffraction grating 129, antireflection coating films 135 are formed on both ends.
The laser body thus constructed is mounded on a heat sink 140 controlled at a constant temperature by a temperature sensor and a Peltier element.
In the three-electrode DFB semiconductor laser device, a case will be considered where a current Is flowing in the electrodes 113b, 113c provided on the current injection regions on both ends and a current Ic flowing in the electrode 113a provided on the current injection region 131 in the center are increased so as to balance with each other to maintain a single-mode oscillation state.
Since the temperature near the active layer 102 rises as the current increases, a red shift of several nanometers can be caused to take place. Because the entire area is an active region, fluctuations in the carrier density at the time of current injection are small, making the line width narrow. However, as it stands now, the response speed of thermal effects is slow, with the result that high-speed wavelength switching as required for optical LANs is difficult.
In addition to thermal effects, carrier effects contribute to the change of wavelength in the multielectrode DFB semiconductor laser device. An increase in the carrier density resulting from an increase in the current causes the refractive index to decrease, leading to a blue shift of oscillation wavelength. Although the response speed of carrier effects is several nanoseconds, a change in the carrier density in the active layer in an oscillating state is so small that the carrier effects alone generally can change the oscillation wavelength as much as 2 nanometers at most. It is well known that the sign of carrier effects is opposite to that of thermal effects and they differ greatly in response speed. These things have been described in, e.g., J. Jacquet et al., "Thermal contribution to wavelength tunability of multielectrode DFB layers," Technical Digest, Optical Fiber Communication (OFC) 91, paper FB4, p. 204.
In the multielectrode DFB semiconductor laser device, a red shift of carrier effects can be effected by suitably setting a bias and other factors. In this case, however, it is impossible to make the absolute value of a wavelength change large. This relationship between carrier effects and thermal effects makes it very difficult to switch wavelengths at high speeds.
This problem will be explained using FIGS. 2 and 3. FIG. 2 illustrates how current Ic flowing in electrode 113a changes with time in the three-electrode DFB semiconductor laser device of FIG. 1. FIG. 3 shows how the oscillation wavelength changes at that time.
Now, it is assumed that at time 0, current Ic is increased substantially and a red shift as great as several nanometers is achieved. Since the response speed of an increase in the carrier density in the active layer 102 resulting from an increase in Ic current is on the order of nanoseconds, the oscillation wavelength makes a blue shift immediately as shown in FIG. 3.
Although most of the input energy resulting from the current increase at the p-n junction in the active layer 102 is spent on the increase of optical energy, part of the energy is converted into heat as a result of an increase in the nonluminescent recombination current. In addition, the increase of current Ic causes an increase in the amount of heat generated by the contact resistance of electrode 113a, and an increase in the amount of heat generated by the resistive component between the ohmic contact layer 136 and the active layer 102. As a result, the temperature of the thermally active layer 102 rises gradually as time passes, which allows the oscillation wavelength to make a red shift as shown in FIG. 3.
Since thermal effects are generally greater than carrier effects, the oscillation wavelength becomes equal to the original oscillation wavelength at time Ta after current Ic has been increased, as shown in FIG. 3. Since then, the red shift further continues. The temperature of the active layer 102 finally becomes stable at an equilibrium temperature determined by the heat generating state and the heat sink temperature. The time required for the temperature to be stable is several milliseconds.
To shorten this time, feedback control is applied to the three-electrode DFB semiconductor laser device of FIG. 1 as shown in FIG. 4.
The laser light 137 emitted from the three-electrode DFB semiconductor laser device passes through a lens system 141 and an optical isolator 142, and enters a beam splitter 143. The beam splitter 143 splits nearly 10% of the optical power to supply it to a Mach-Zehnder interferometer 144 acting as a frequency discriminator, and directs the remaining optical power to an emission fiber pigtail 149. The Mach-Zehnder interferometer 144 outputs a signal of a level proportional to the change of wavelength. The level is sensed by a photodiode 145, and is used by a wavelength controller 146 as an error signal for control of Ic. Here, the time constant of the parts excluding the laser in the feedback system is shorter than the response time of thermal effects.
Now, it is assumed that a reference signal is externally supplied to the wavelength controller 146 to change the reference value at the wavelength controller 146, thereby making a red shift of oscillation wavelength. The wavelength controller 146 controls current Ic so that the difference (.lambda..sub.ref -.lambda..sub.0) between the wavelength .lambda..sub.ref corresponding to the new reference value and the current wavelength .lambda..sub.0 may be smaller. Namely, the wavelength controller 146 increases current Ic so that wavelength .lambda..sub.0 may be larger.
At this time, because the oscillation wavelength first undergoes a blue shift due to carrier effects, the wavelength .lambda..sub.0 becomes not larger but smaller. Consequently, the error signal increases and the wavelength controller 146 tends to increase current Ic further.
In this way, the wavelength controller 146 based on thermal effects continues increasing current Ic. In the meantime, when the error signal has begun to decrease due to thermal effects, the wavelength controller 146 begins to decrease current Ic to the contrary. However, a decrease in Ic results in a red shift momentarily, giving rise to an excessive response.
As a result, ordinary PID control cannot control the oscillation wavelength successfully, and it is difficult to stabilize the oscillation wavelength to a specified value in a short time. Particularly, a momentary response is of the opposite phase, so that differential control cannot be effected properly. For this reason, a complicated feedback system is required.
At any rate, since a wavelength that can be shifted only by carrier effects is smaller than that by thermal effects, slow-response thermal effects alone can change the wavelength greatly. That is, the time constant of the feedback system is limited by the response time of slow thermal effects.
FIG. 5 shows a schematic structure of another example of the three-electrode DFB semiconductor laser device.
In the figure, numeral 151 indicates an n-type InP substrate. On the n-type InP substrate 151, an n-type InP cladding layer 153 is formed. On top of a narrowed portion of the n-type InP cladding layer 153, a striped strained quantum well active/optical waveguide layer 154 of approximately 1 .mu.m width is formed. The narrowed portion of the n-type InP cladding layer 153 is buried with a p-type InP layer 157 and an n-type InP layer 158.
On the strained quantum well active/optical waveguide layer 154, a p-type InP cladding layer 155 and a p-type ohmic contact layer 156 are formed in sequence. The p-type InP cladding layer 156 is provided with a p-side ohmic electrode 159, whereas the n-type InP substrate 151 is provided with an n-side ohmic electrode 160.
Although not shown in the figure, the p-type InGaAsP ohmic contact layer 156 and the p-side ohmic electrode 159 are divided into three pieces, a central region and regions on both ends, in each of which current is allowed to flow independently. Furthermore, a single-mode DFB oscillation is realized by a first-order diffraction grating (not shown) formed on the strained quantum well active/optical waveguide layer 154.
With the three-electrode DFB semiconductor laser device thus constructed, when currents flowing in the three regions are increased in a balanced manner so as to maintain a single-mode oscillating state, the temperature near the strained quantum well active/optical waveguide layer 154 rises. This therefore causes a red shift of several nanometers. In addition, because all the regions are active, fluctuations in the carrier density at the time of current injection are small, consequently making the line width narrow.
However, as it stands now, when the semiconductor laser device is applied to an optical FDM system where many wavelength channels are switched at high speeds, this results in a very inefficient system because the response speed of thermal effects is slow and therefore it takes a long time to switch wavelengths.
As mentioned above, a conventional optical apparatus using a conventional multielectrode DFB semiconductor laser has a problem: although the spectral line width can be kept narrow over a wide wavelength changing range, it takes a long time to switch channels.
Additionally, an optical transmission apparatus using a conventional multielectrode DBR semiconductor laser has a problem: although the channel switching time is short, the wavelength changing range over which the spectral line width can be kept narrow is narrow.